The advantages of two-cycle engines are well known. They are simple, have a high power/weight ratio, can be manufactured at a low cost and are very reliable. These characteristics have made the two-cycle engine the preferred power source for hand held appliances such as chain saws, line trimmers, leaf blowers and the like. However, the necessity for ensuring complete combustion and minimization of scavenging losses of the engine present significant problems.
Most of the modern two-cycle engines employ three basic types of scavenging systems: Loop scavenging (FIG 1a), cross scavenging and uniflow scavenging (FIG. 2). The loop scavenging system, being the most popular due to its simplicity and effectivity. The scavenging ports direct a stream of air/fuel mixture into the cylinder, creating a loop like flow pattern, aiming to evacuate the remaining gases left from the combustion cycle. Despite the numerous improvements implemented through time in the loop scavenging system since its invention by Schnuerle in 1926, an unavoidable portion of unburned fuel is always released into the atmosphere as scavenging losses. This reduces the fuel efficiency of the engine and creates atmospheric pollution.
Pending and existing exhaust emission regulations imposed by the EPA and CARB on non-road equipment up to 19 Kw including lawn and garden equipment powered by internal combustion engines, strongly demand reduction in noxious substances such as hydrocarbons, nitrous oxides and carbon monoxide, in exhaust gas discharged mainly by two-cycle engines used on power tools and other lightweight applications.
In order to meet such existing and pending air pollution exhaust emission regulations, for such hand held two-cycle engines, much effort and expense has been directed in the last several years towards improving scavenging and fuel delivery systems for such engines to enable the same to meet such stricter exhaust pollution requirements, especially in regard to the unburned hydrocarbon (HC) component. In this field, the major hurdle has been to achieve this result at an affordable cost to the end user of such relatively low cost equipment, while also insuring that such engine improvements do not compromise the easy portability requirements for such engine powered handheld appliances and equipment.
New generations of lightweight four-cycle engines with low hydrocarbon emissions are among the technologies being developed for powering handheld portable tools. Their manufacturing cost, in-use emissions deterioration, serviceability and low power/weight ratio are still problems to resolve. An example of this technology is illustrated by U.S. Pat. No. 5,558,057.
Catalytic converters, fuel injection, uniflow scavenging and stratified scavenging are among the technologies aimed to reduce exhaust emissions in modern two-cycle engines.
Catalytic converters are well known from automotive applications as an efficient method to reduce exhaust emissions. The hydrocarbon reduction is a result of a chemical reaction that produces oxidation of exhaust gases. Unfortunately, the catalyst materials deteriorates with use, do not completely eliminates the hydrocarbon emissions and generates unwanted amounts of heat, factors that are not very appealing in small engines.
Uniflow scavenging is another method used to improve the fuel efficiency and to reduce the scavenging losses incurred in loop scavenged two-cycle engines. Uniflow engines were successfully used in the 30's on automotive and diesel engines by Trojan, Garelli, DKW, Puch, TWN and EMC. Longer scavenging loop and clever asymmetric geometry allowed the port timing of some split singles to be juggled so that the exhaust closes before charging has finished, which all helped to keep the fresh mixture out of the exhaust increasing the fuel efficiency. These advantages of uniflow scavenging are used for reducing exhaust emissions in modern applications. Examples of this method are provided by U.S. Pat. Nos. 4,079,705, 5,722,355 and 5,758,611
Another well-known approach successfully used to reduce scavenging losses is direct fuel injection systems. Thanks to recent electronic technology developments, electronic fuel injection is presently widespread as the preferred fuel delivery system in automotive applications. Unfortunately, this technology has not been commercially developed in low cost lightweight applications due to the electrical hardware required and its associated high cost. Also, the complexity of a fuel injection system to manage the small fuel volumes required at idle and at full throttle conditions, has remained as another serious obstacle to successfully implement direct fuel injection systems in hand held appliances powered by two-cycle engines.
Long before electronic fuel injection was technologically possible, mechanical fuel injection systems were widely used on diesel engines and on high performance gasoline engines. Many attempts have been made to adapt mechanical fuel injection to small engines, but cost and functional factors have been significant barriers to these systems.
During last century, the earliest efforts with regard to the development of a mechanical fuel injected two-cycle engine, was the Clerk engine. This engine used a mechanical pump to transfer air/fuel mixture to a working cylinder. Since then, other engine inventors followed the same basic Clerk's principles in their engines. Most of these early inventions were originated from diesel engines concepts where high compression ratios are necessary. The U.S. Pat. No. 607,276 by the Joseph Reid Gas Company issued in 1898 describes an engine with a pump cylinder and a power cylinder used for oil well service, where the pump cylinder was used to transfer natural gas mixed with air into the working cylinder. Another early application of volumetric fuel pumps in two-cycle engines were the supercharged racing engines by DKW and Schilha in the early 1900's.
The U.S. Pat. No. 1,168,425 by Rosenhagen issued in 1916 describes a typical example of prior art engines using a volumetric pump to transfer the air/fuel mixture into the working cylinder. This engine uses timing differences based on the radial positioning of a pump piston in relation to a power piston to create anticipated upward motion of the pump piston thus creating a pressure differential between both cylinders. Complicated valving and fuel passages, low speed, large air/fuel paths, high pumping losses and lack of lubricating means prevented the success of this invention seeking improved volumetric efficiencies.
Another examples of use of volumetric pumps to transfer a combustible mass to a working cylinder is described on patents by Houyez (France 908,916), Silander (Belgium 515,577), Kerrebrok (U.S. Pat. No. 4,506,634), Voisin (France 1,084,655) and Lepore (Italy 434,901) among others.
The aforementioned prior art on two-cycle engines with volumetric pumping systems was intended for low speed, large engines, capable of absorbing large pumping losses, high levels of vibration and with a multiplicity of components not tolerated by small hand held engines with low levels of power and inexpensive manufacturing. These prior art engines did not succeed due to the competitiveness of loop scavenged two-cycle engines in an era where exhaust emissions were unimportant. Therefore, there is a need in the art for a small high performance two-cycle engine with very low hydrocarbon emissions that can be successfully fabricated with current mass production methods at a cost affordable for such inexpensive applications.
A modern example of air assisted mechanical fuel injection systems using volumetric fuel pumps is provided by the FAST system. A volumetric pump driven by a secondary crankshaft introduces a rich air/fuel mixture into a power cylinder. The Italian manufacturer Piaggio successfully uses this system to reduce exhaust emissions in motor scooters. As it will be learned further in the description of this invention, the engine object of the present invention uses equivalent physical principles to those used by the FAST system to gain volumetric efficiency and reduced hydrocarbon emissions. The engine object of the present invention provides the same effects, but thanks to the use of a greatly simplified mechanical system, it allows a lightweight and compact construction as well as reduced mechanical losses that allows its utilization on portable tools.
It is obvious to the person skilled in the art, that the prior art of air assisted mechanical fuel injection in two-cycle engines, often has a complex and bulky construction not desirable for lightweight applications and hand held portable tools where compactness, low weight, low cost and low emissions are the dominating factors.